Depends on the person measuring. Myself (as I build chassis's) measure the shoulder hoop at the tubing centerline where the bend starts. So when I say that the shoulder hoop is on 21" centers using 1.5" dia tubing, the inside width without the seat is 19.5".
Back in the day when drivers (back from war) were 138#, the suits were tin-foil, and the helmets were there to keep the hair out of your eyes, 19" (on center) was the standard, Now with bigger drivers, the SFI 15/20 suits and the larger helmets, along with a HANS, 21" (on center) has become the standard along with the shoulder hoop made from 2- 6" radius bends with a straight section between them verses the full radius bend an no straight. If you are big, try and avoid the "round back" chassis as it rolls/hunches your shoulders in/up too much and really confinds your movements and the Hans does not fit properly, A square back design allows your back to fit flat against the chassis/seat for more comfort and movement
